The high temperatures and strenuous oxidizing conditions existing in the combustor, turbine and exhaust portions of a gas turbine engine under certain conditions have resulted in damage or reduced component life. For example, thermal fatigue cracking and other thermal distress have resulted in removal and replacement or extensive repair of metal components.
Because of their high temperature capabilities, ceramic materials have been evaluated for use as members in hot operating portions of gas turbine engines. However, a characteristic of such ceramic members, as well as of members made from some very high temperature capability metallic materials and ceramic-metal composites, is low ductility, thus classifying them as being brittle and generally of low impact resistance or strength. There is a high tendency in such members for the initiation and propagation of cracks which can occur from mechanical constraints imposed by metal backing or holding members due to thermal expansion mismatch. In order to utilize a member made of a ceramic or similarly characterized material in gas turbine engines or other high temperature operating apparatus, means are required to retain such a member and to cushion or overcome the constraints which might result from thermal mismatch with a holding member.
In order to minimize or reduce the mismatch in coefficients of thermal expansion between metal supports and ceramic members, the prior art has reported such arrangements as the use of graded layers between the metal and the ceramic member, the distribution of metals within the ceramic structure to form a composite, or the use of fibrous metal felt as a bonding layer between the metal and the ceramic member. All of these approaches show unsatisfactory thermal fatigue resistance as evidenced by the spallation or separation of the ceramic from the metal backing in thermal shock tests. The other approach has been the use of ductile metal foil such as Pt or a cobalt base alloy commercially available as L605 alloy. Such foils deform, extrude and oxidize, and thereby fail to provide compliancy for a long period of time.